In recent years, an IP network such as a wired LAN (Local Area Network) or a wireless LAN is widespread more and more to become an infrastructure for daily life of people. Price reduction of a network I/F (interface) is also progressing.
Also, with regard to peripherals locally connected to a computer in the past, it has become possible to control peripherals via a computer and an IP network under the current circumstances in which an IP network speed has become high enough to be compatible with sending/receiving of control data.
Under such circumstances, a device server system that enables a USB (Universal Serial Bus) device, i.e. a computer peripheral to be operated through an IP network by a computer connected to said IP network is known. (Refer to Patent Literature 1, Non-Patent Literature 1)
In the device server system, a USB data such as a control command issued by a USB device driver of a client is converted to an IP packet form (encapsulation), and this IP packet is transferred to a device server from the client. The device server that receives the IP packet retrieves a USB data such as a control command contained in the received IP packet (de-capsulation) and transfers to a USB device connected locally to the device server itself after converting the USB data into the one in USB communication mode.
Communication between the device server and the USB device is in accordance with the USB standard for communication. For example, in a case of communication from the client to the USB device, the device data is transferred from the client to the USB device via the device server and at the same time the client receives response regarding the USB device status whenever necessary via the device server.
When the USB device is a speaker, the client transfers streaming data such as sound which requires being real time to the USB device. In such a case, by performing the data transfer by the isochronous method (isochronous transfer) that ensures a minimum data transfer volume per a definite period of time, the data can be transferred without a pause.
When the isochronous transfer is performed between a client and a USB device via a device server, the client and the USB device is not locally connected but connected via a network, which leads to elevated network traffic and consequently the data transfer speed among network segments is degraded. Even if that is the case, it is necessary to maintain the minimum data transfer volume that should be guaranteed by the isochronous transfer.
A control method of a device server, in the course of internal processing of the device server, that transfers response information from a USB device to a device driver of the client, in a mode of a dummy response to the isochronous transfer request is known.
(Patent Literature 2)
According to the control method disclosed in Patent Literature 2, the client can maintain the minimum data transfer volume guaranteed by the isochronous output transfer because stagnation of the processing of the next isochronous transfer request due to the delay (waiting for reception) of response information from the USB device can be avoided.
However, according to the control method of the device server disclosed in Patent Literature 2, the device server transfers to the device driver of the client dummy response data disguised as response information from the USB device even when the device server does not receive response information from the USB device within a predetermined time, which leads to an undesirable situation in which the client sends out the next isochronous transfer request while the device server cannot receive this request and cannot deliver the request to the device when the network itself is in an unrecoverable status (fatal status).